Coatable conductive layer

ABSTRACT

The invention relates to a patternable coatable electrically conductive layer comprising a fluid-coated electrically conductive material, wherein the fluid-coated electrically conductive material has sufficient conductivity to induce an electric field strong enough to change the optical state of a light modulating material and a display comprising a substrate, at least one patternable coatable electrically conductive layer comprising a fluid-coated electrically conductive material, wherein said fluid coated electrically conductive material has sufficient conductivity to induce an electric field strong enough to change the optical state of a light modulating material which has a first and a second field-switched stable optical state, and an imaging layer comprising said light modulating material disposed over said at least one patternable fluid-coated electrically conductive layer. The invention also relates to a method for making a coatable electrically conductive layer and a method for making a display with a coatable electrically conductive layer.

FIELD OF THE INVENTION

The present invention relates to forming coatable conductive electrodesfor flexible displays.

BACKGROUND OF THE INVENTION

Currently, information is displayed using assembled sheets of papercarrying permanent inks or displayed on electronically modulatedsurfaces such as cathode ray displays or liquid crystal displays.Printed information displayed in these manners cannot be changed.Devices that allow for the modification of information, such aselectrically updated displays, are often heavy and expensive.Information may also be applied to sheet materials via magneticallywritten areas, for example, to carry ticketing or financial information.Such magnetically written data, however, is not visible.

Media systems exist that maintain electronically changeable data withoutpower. Such system can be electrophoretic (E-ink), Gyricon, or polymerdispersed cholesteric materials. An example of such electronicallyupdateable displays can be found in U.S. Pat. No. 3,600,060, which showsa device having a coated, then dried emulsion of cholesteric liquidcrystals in aqueous gelatin to form a field-responsive, bistabledisplay. U.S. Pat. No. 3,816,786 also discloses a layer of encapsulatedcholesteric liquid crystal responsive to an electric field. Theelectrodes in the patent can be transparent or non-transparent andformed of various metals or graphite. It is disclosed that one electrodemust be light absorbing, and it is suggested that the light absorbingelectrode be prepared from paints containing conductive material such ascarbon.

Fabrication of flexible, electronically written display sheets isdisclosed in U.S. Pat. No. 4,435,047. A substrate supports a firstconductive electrode, one or more layers of encapsulated liquidcrystals, and a second electrode of electrically conductive ink. Theconductive inks form a background for absorbing light, so that theinformation-bearing display areas appear dark in contrast to backgroundnon-display areas. Electrical potential applied to opposing conductiveareas operates on the liquid crystal material to expose display areas.Because the liquid crystal material is nematic liquid crystal, thedisplay ceases to present an image when de-energized, that is, in theabsence of a field. A first flexible substrate is patterned which iscoated. A second pre-patterned substrate is bonded over the coating.

U.S. Pat. No. 5,251,048 discloses a light-modulating cell having apolymer-dispersed chiral-nematic liquid crystal. The chiral-nematicliquid crystal has the property of being electrically driven between aplanar state, reflecting a specific visible wavelength of light, and afocal-conic state, transmitting forward scattering light. Chiral-nematicliquid crystals, also known as cholesteric liquid crystals, potentiallyin some circumstances have the capacity of maintaining one of multiplegiven states in the absence of an electric field. Black paint can beapplied to the outer surface of a rear substrate to provide alight-absorbing layer forming a non-changing background outside of achangeable display area defined by the intersection of segment lines andscanning lines. A first glass substrate is patterned. A second patternedglass substrate is fixable spaced from the first substrate. The cavityis filled with liquid crystal.

U.S. Pat. No. 6,025,952 discloses a sheet having a light sensitive layerthat can be patterned to form conductors that respond to electricalsignals to operate on a light-modulating layer. The light-modulatinglayer is polymer dispersed liquid crystal and the light patternedconductor layer is silver halide layer. A sheet made according to theinvention requires a light exposure step and subsequent silver halideprocessing. Silver halide processing requires repeated chemicaldiffusion processes.

U.S. Pat. No. 6,236,442 discloses a display sheet with a metallicconductive layer over a light modulating layer. The conductive layer isformed by vacuum depositing a continuous metallic layer and laserpatterning the metallic layer to form segment electrodes. However,vacuum deposited conductive layers are expensive and fragile.

U.S. Pat. No. 6,394,870 discloses directly depositing opaque conductiveink in an image wise pattern by screen-printing. Screen-printing issensitive to ink formulation, adhesion and printing process parameters.The inks require secondary processes, such as heating or ultra-violetcuring to set. The materials can contain polymeric binders that arepersonally and environmentally harmful. Screens utilizing thistechnology have limited life and require periodic cleaning. Suchprocesses can take many seconds to deposit the material. It would beuseful to provide environmentally safe materials with a fast, drypatterning process.

U.S. Pat. Application No. 2003/0174264 A1 discloses that polymerdispersed liquid crystal materials can be coated on photographicmachinery as part of a multiple layer coating. Such coatings requiresubsequent application of conductors. It would be useful to apply aconductive coating simultaneous with the deposition of the polymerdispersed liquid crystal layer.

PROBLEM TO BE SOLVED

There is a need for a display using polymer-dispersed cholesteric liquidcrystal material having a conductor that can be applied without the useof vacuum coatings. It would be of further use if such a layer could bepatterned without the need for diffusive solution baths or patterned inkprinting.

SUMMARY OF THE INVENTION

The present invention relates to a patternable coatable electricallyconductive layer comprising a fluid-coated electrically conductivematerial, wherein the electrically conductive material has sufficientconductivity to induce an electric field strong enough to change theoptical state of a light modulating material. The present invention alsorelates to a display comprising a substrate; at least one patternablecoatable electrically conductive layer comprising a fluid-coatedelectrically conductive material, wherein the fluid-coated electricallyconductive material has sufficient conductivity to induce an electricfield strong enough to change the optical state of a light modulatingmaterial, and an imaging layer comprising the light modulating materialdisposed over the at least one patternable fluid-coated electricallyconductive layer, wherein the light modulating material has a first anda second field-switched stable optical state. In addition, the inventionrelates to a method for making a coatable electrically conductive layercomprising providing a substrate and coating thereon an electricallyconductive layer comprising a fluid-coated electrically conductivematerial, wherein the fluid-coated electrically conductive material hassufficient conductivity to induce an electric field strong enough tochange the optical state of a light modulating material and a method formaking a display comprising providing a substrate, applying a patternedconductive layer thereto, coating a light modulating layer onto theconductive layer and coating thereon a coatable electrically conductivelayer comprising a fluid-coated electrically conductive material,wherein the fluid-coated electrically conductive material has sufficientconductivity to induce an electric field strong enough to change theoptical state of a light modulating material.

ADVANTAGEOUS EFFECT OF THE INVENTION

The present invention includes several advantages, not all of which areincorporated in a single embodiment. The present invention eliminatesvacuum deposited conductors, printing machinery and chemicaldevelopment. The invention has the advantage that a liquid coating canbe applied over an optical layer to create a patterned conductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective of a display in accordance with the presentinvention.

FIG. 2 is a schematic sectional view of a chiral nematic material,respectively, in a planar and focal-conic state responding to incidentlight.

FIG. 3 is a first sectional view of fluidic deposition of a conductivelayer.

FIG. 4 is a second sectional view of fluidic deposition of a conductivelayer.

FIG. 5 is a first side sectional view of a display having a fluiddeposited conductive layer in the wet and dried state.

FIG. 6 is a second side sectional view of a display having a fluiddeposited conductive layer in the wet and dried state.

FIG. 7 is a side sectional view of a dried, fluid deposited conductorbeing etched by a laser beam.

FIG. 8 is a rear view of a sheet in accordance with the presentinvention having a patterned first conductor.

FIG. 9 is a rear view of a sheet in accordance with the presentinvention having a polymer-dispersed cholesteric liquid-crystal layer, adark layer, and a fluid deposited conductive layer.

FIG. 10 is a rear view of a sheet in accordance with the presentinvention having exposed first conductors.

FIG. 11 is a rear view of a display in accordance with the presentinvention having second conductors etched into fluid depositedconductive layer.

FIG. 12 is a section view of a display in accordance with the presentinvention attached to a circuit board.

FIG. 13 is a front view of a display in accordance with the presentinvention connected to electric drive means.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a fluid-coated conductive layer,preferably for use in liquid crystal display applications. Thefluid-coated conductive layer preferably has sufficient conductivity toinduce an electric field across a layer positioned next to the coatedconductive layer. In a preferred embodiment, the fluid-coated conductivelayer has sufficient conductivity to induce an electric field strongenough to change the optical state of the light modulating material.

As referred to herein, a coated layer refers to a thin layer covering asubstrate or other layer. Coating refers to the act of applying acoating to a substrate or other layer, putting a coat on a substrate orother layer, or covering a surface. Coatings apply an unpatternedcontinuum across a specific area. This is different that printing, asprinting is defined to mean producing written or print characters, whichare multiple patterned areas, produced by means of or the result ofprinting or producing visible indication made on a surface.

In this invention, the coated conductors are conductive materials thatare preferably fluid coated, dried and patterned. Preferably, theconductive material in the fluid-coated conductive layer has aconductivity of less than 10⁴ ohms/sq. In the specific embodiment, thecoated conductive layer is formed into discrete patterns using actinicradiation, preferably a laser. Fluid coating refers to the applicationof a material in a solvent or dispersing medium, which solvent ordispersing medium is later removed to produce the final applied layer ofmaterial. Preferably, the fluid is water or an organic solvent.

In one embodiment, fine conductive particles are suspended in the fluidwith a binding agent, also referred to herein as a binder. Particles inthe invention must be fine enough to stay in suspension during a coatingprocess, and provide a sufficiently conductive surface with a coatingthickness thin enough to be laser etched when used to form secondconductors without damage to first transparent conductors. In apreferred embodiment, the fluid-coated material may be fine metallicparticles or flakes suspended in a gelatin solution. Preferably, theparticles have a diameter of less than 1 micron, and more preferably, adiameter of less than 50 nm. However, any size particle may be utilized,provided it is able to remain suspended in the coating.

The fine conductive particles useful in the application may includesilver, silver plated copper particles, nickel, gold, palladium, carbonor combinations of the materials. Preferred materials may includeparticles, such as silver, nickel or chrome. The material selectedshould resist oxidation and be low cost. Precious metals such aspalladium and gold resist oxidation, but are expensive. Fine silverparticles are preferred in the embodiment because silver is a lower costprecious metal, silver is highly conductive and oxide coatings on silverare conductive.

Fine particulate silver can be formed by flame spray separation,continuous vapor deposition or chemical precipitation. Chemicalprecipitation methods can include forming fine silver-halide grains,that are developed and separated from the resulting salts to form silverparticles having a diameter of less between 0.5 and 2.0 micron,preferably less than 2.0 microns, more preferably, less than 1 micron.The silver particles may also preferably measure less than 10 cubicmicrons across the major length. Materials formed by chemicalprecipitation require extensive chemical processing, and aresignificantly more expensive than directly formed particles. Silver inthat size range precipitates slowly enough for machine coating and isrelatively inexpensive.

Alternatively, the fluid can contain a soluble or insoluble polymericconductor such as doped polypyrrole in solution with the solvent. Inanother embodiment, the coating solution contains an organic conductor.In another embodiment, the coating solution is an organic conductor,such as doped polypyrrole along with conductive particles in suspensionwith the dissolved organic conductor. Coated conductive layers can alsobe formed by printing a transparent organic conductor such as PEDT/PSS,PEDOT/PSS polymer, which materials are sold as Baytron® P by Bayer AGElectronic Chemicals.

The binding agent or binder is preferably polymeric. The binder may be ahydrophilic binder and may include both naturally occurring substancessuch as proteins, protein derivatives, cellulose derivatives (e.g.cellulose esters), gelatins and gelatin derivatives, polysaccaharides,casein, and the like, and synthetic water permeable colloids such aspoly(vinyl lactams), acrylamide polymers, poly(vinyl alcohol) and itsderivatives, hydrolyzed polyvinyl acetates, polymers of alkyl andsulfoalkyl acrylates and methacrylates, polyamides, polyvinyl pyridine,acrylic acid polymers, maleic anhydride copolymers, polyalkylene oxide,methacrylamide copolymers, polyvinyl oxazolidinones, maleic acidcopolymers, vinyl amine copolymers, methacrylic acid copolymers,acryloyloxyalkyl acrylate and methacrylates, vinyl imidazole copolymers,vinyl sulfide copolymers, and homopolymer or copolymers containingstyrene sulfonic acid. Gelatin is preferred for compatibility withliquid crystal materials also dispersed in gelatin.

A light-modulating layer is defined herein as an electrical fieldresponsive material that changes optical state in response to anelectrical field. Possible materials can be electrophoretic materials orliquid crystal materials. Liquid crystal materials can be nematicmaterials, polymer dispersed liquid crystals. Some of these materialsmaintain multiple optical states in the absence of an electrical field.These materials can be dispersed in polymer using a variety of means toform a polymer dispersed electro-optical layer requiring a secondelectrode. In a preferred embodiment, light modulating material ischolesteric liquid crystal material, such as those disclosed in U.S.Pat. No. 5,695,682, the disclosure of which is incorporated byreference.

As used herein, the phase a “liquid crystal display” (LCD) is a type offlat panel display used in various electronic devices. At a minimum, anLCD comprises a substrate, at least one conductive layer and a liquidcrystal layer. The sheets of polarizing material may comprise asubstrate of glass or transparent plastic. The LCD may also includefunctional layers. In another embodiment, a display comprises asubstrate, at least one transparent conductor, an imaging layercomprising a light modulating material disposed over the conductor,wherein the light modulating material has the property of having a firstand second field-switched stable optical states. In another embodiment,the display further comprises a second conductor.

Cholesteric liquid crystal materials may be made using highlyanisotropic nematic liquid crystal mixtures and adding a chiral dopingagent to provide helical twist in the planes of the liquid crystal tothe point that interference patterns are created that reflect incidentlight. Chiral nematic liquid crystal refers to the type of liquidcrystal having finer pitch than that of twisted nematic andsuper-twisted nematic used in commonly encountered LC devices. Chiralnematic liquid crystals may be used to produce bi-stable or multi-stabledisplays. Application of electrical fields of various intensity andduration can be employed to drive a chiral-nematic (cholesteric)material into a reflective state, to near-transparent or transmissivestate, or an intermediate state. These materials have the advantage ofhaving first and second optical states that are both stable in theabsence of an electrical field. These devices have significantly reducedpower consumption due to their non-volatile “memory” characteristic.Liquid crystals can be nematic (N), chiral nematic (N*), or smectic,depending upon the arrangement of the molecules in the mesophase. Chiralnematic liquid crystal (N*LC) displays are typically reflective, thatis, no backlight is needed, and can function without the use ofpolarizing films, alignment layers or a color filter.

Since such displays do not require a continuous driving circuit tomaintain an image, they consume significantly reduced power wheninfrequently addressed. Chiral nematic displays are bistable in theabsence of a field; the two stable textures are the reflective planartexture and the weakly scattering focal conic texture. In the planartexture, the helical axes of the chiral nematic liquid crystal moleculesare substantially perpendicular to the substrate upon which the liquidcrystal is disposed. In the focal conic state the helical axes of theliquid crystal molecules are generally randomly oriented. Adjusting theconcentration of chiral dopants in the chiral nematic material modulatesthe pitch length of the mesophase and, thus, the wavelength of reflectedradiation. Chiral nematic materials that reflect infrared radiation andultraviolet have been used for purposes of scientific study. Commercialdisplays are most often fabricated from chiral nematic materials thatreflect visible light. Some known LCD devices include chemically-etched,transparent, conductive layers overlying a glass substrate as describedin U.S. Pat. No. 5,667,853, incorporated herein by reference.

In one embodiment, a chiral-nematic liquid crystal composition may bedispersed in a polymer matrix. Such materials are referred to as“polymer-dispersed liquid crystal” materials or “PDLC” materials. Suchmaterials can be made by a variety of methods.

Liquid crystals have bulk properties equivalent to fluids at roomtemperature. Polymer dispersed liquid crystal domains are areas ofliquid crystal existing in a matrix of polymeric material with morerigid properties. In one case, liquid crystal material exists asindividual droplets in a rigid polymer matrix. In another case, liquidcrystal material is suspended in a mesh of polymeric threads. The PDLCcan be formed by conventional emulsions formed wherein one of twoimmiscible fluids is a liquid crystal. The second fluid can contain apolymer in solution. The emulsion can be coated and dried to create alayer having individual droplets of liquid crystal supported by a driedpolymer matrix. Alternatively, a monomer can be dissolved in the liquidcrystal material and polymerized to create separated droplets of liquidcrystal material or ordered threads of polymer within the liquid crystalmedium.

In the preferred embodiment, domains are formed using the limitedcoalescence technique described in as disclosed in U.S. Pat. Nos.6,556,262 and 6,423,368, incorporated by reference. Preferably, thedomains are flattened spheres and have on average a thicknesssubstantially less than their length, preferably at least 50% less. Morepreferably, the domains on average have a thickness (depth) to lengthratio of 1:2 to 1:6. The flattening of the domains can be achieved byproper formulation and drying in a viscous state. The domains preferablyhave an average diameter of 2 to 30 microns. The imaging layerpreferably has a thickness of 10 to 150 microns when first coated and 2to 20 microns when dried. In a preferred embodiment of the invention,the display device or display sheet has simply a single imaging layer ofliquid crystal material along a line perpendicular to the face of thedisplay, preferably a single layer coated on a flexible substrate. Suchas structure, as compared to vertically stacked imaging layers eachbetween opposing substrates, is especially advantageous for monochromeshelf labels and the like. Structures having stacked imaging layers,however, are optional for providing additional advantages in some case.

The flattened domains of liquid crystal material can be defined ashaving a major axis and a minor axis. In a preferred embodiment of adisplay or display sheet, the major axis is larger in size than the cell(or imaging layer) thickness for a majority of the domains. Such adimensional relationship is shown in U.S. Pat. No. 6,061,107, herebyincorporated by reference in its entirety.

The contrast of the display may be degraded if there is more than asubstantial monolayer of N*LC domains. In a preferred embodiment, theliquid crystalline material comprises a substantial monolayer. The term“substantial monolayer” is defined by the Applicants to mean that, in adirection perpendicular to the plane of the display, there is no morethan a single layer of domains sandwiched between the electrodes at mostpoints of the display (or the imaging layer), preferably at 75 percentor more of the points (or area) of the display, most preferably at 90percent or more of the points (or area) of the display. In other words,at most, only a minor portion (preferably less than 10 percent) of thepoints (or area) of the display has more than a single domain (two ormore domains) between the electrodes in a direction perpendicular to theplane of the display, compared to the amount of points (or area) of thedisplay at which there is only a single domain between the electrodes.

The amount of material needed for a monolayer can be accuratelydetermined by calculation based on individual domain size, assuming afully closed packed arrangement of domains. (In practice, there may beimperfections in which gaps occur and some unevenness due to overlappingdroplets or domains.) On this basis, the calculated amount is preferablyless than about 150 percent of the amount needed for monolayer domaincoverage, preferably not more than about 125 percent of the amountneeded for a monolayer domain coverage, more preferably not more than110 percent of the amount needed for a monolayer of domains.Furthermore, improved viewing angle and broadband features may beobtained by appropriate choice of differently doped domains based on thegeometry of the coated droplet and the Bragg reflection condition.

Liquid crystal domains may be preferably made using a limitedcoalescence methodology, as disclosed in U.S. Pat. Nos. 6,556,262 and6,423,368, incorporated herein by reference. Limited coalescence isdefined as dispersing a light-modulating material below a given size,and using coalescent limiting material to limit the size of theresulting domains. By use of the term “uniform domains”, it is meantthat domains are formed having a domain size variation of less than 2:1.Limited domain materials may have improved optical properties.

Limited coalescence is defined as providing an immiscible, fieldresponsive light-modulating material along with a quantity of colloidalparticles dispersed in an aqueous system and blended to form adispersion of field-responsive, light-modulating material below acoalescence size. When the dispersion, also referred to herein as anemulsion, is dried, a coated material is produced which has a set ofuniform domains having a plurality of electrically responsive opticalstates. The colloidal solid particle, functioning as an emulsifier,limits domain growth from a highly dispersed state. Uniformly sizedliquid crystal domains are created and machine coated to manufacturelight-modulating, electrically responsive sheets with improved opticalefficiency.

Specifically, a liquid crystal material may be dispersed an aqueous bathcontaining a water-soluble binder material such as de-ionized gelatin,polyvinyl alcohol (PVA) or polyethylene oxide (PEO). Such compounds aremachine coatable on equipment associated with photographic films.Preferably, the binder has a low ionic content, as the presence of ionsin such a binder hinders the development of an electrical field acrossthe dispersed liquid crystal material. Additionally, ions in the bindercan migrate in the presence of an electrical field, chemically damagingthe light-modulating layer. The liquid crystal/gelatin emulsion iscoated to a thickness of between 5 and 30 microns to optimize opticalproperties of light modulating layer. The coating thickness, size of theliquid crystal domains, and concentration of the domains of liquidcrystal materials are designed for optimum optical properties.

A promoter material, such as a copolymer of adipic acid and2-(methylamino)ethanol, is added to the aqueous bath to drive thecolloidal particles to the liquid-liquid interface. The liquid crystalmaterial is dispersed using ultrasound to create liquid crystal domains.When the ultrasound energy is removed, the liquid crystal materialcoalesced into domains of uniform size.

Domains of a limited coalescent material maintain their uniform sizeafter the addition of the surfactant and after being machine coated.There are few, if any, domains outside narrow limits on a mean dropletsize having undesirable electro-optical properties within the driedcoatings produced by the limited coalescence method. Conventionallydispersed cholesteric materials contain parasitic domains, which reflectlight in wavelengths outside the wavelengths reflected by thecholesteric material. Limited coalescent dispersions have reducedreflection in other wavelengths due to the elimination of parasiticdomains. The increased purity of color is important in the developmentof full color displays requiring well-separated color channels to createa full-color image. Limited coalescent cholesteric materials providepurer light reflectance than cholesteric liquid crystal materialdispersed by conventional methods. Such materials may be produced usingconventional photographic coating machinery.

In order to provide suitable formulations for applying a layercontaining the liquid crystal domains, the dispersions are combined witha hydrophilic colloid, gelatin being the preferred material. Surfactantsmay be included with the dispersion prior to the addition of gelatin inorder to prevent the removal of the particulate suspension stabilizingagent from the droplets. This aids in preventing further coalescence ofthe droplets.

As for the suspension stabilizing agents that surround and serve toprevent the coalescence of the droplets, any suitable colloidalstabilizing agent known in the art of forming polymeric particles by theaddition reaction of ethylenically unsaturated monomers by the limitedcoalescence technique can be employed, such as, for example, inorganicmaterials such as, metal salt or hydroxides or oxides or clays, organicmaterials such as starches, sulfonated crosslinked organic homopolymersand resinous polymers as described, for example, in U.S. Pat. No.2,932,629; silica as described in U.S. Pat. No. 4,833,060; copolymerssuch as copoly(styrene-2-hydroxyethyl methacrylate-methacrylicacid-ethylene glycol dimethacrylate) as described in U.S. Pat. No.4,965,131, all of which are incorporated herein by reference. Silica isthe preferred suspension stabilizing agent.

Suitable promoters to drive the suspension stabilizing agent to theinterface of the droplets and the aqueous phase include sulfonatedpolystyrenes, alginates, carboxymethyl cellulose, tetramethyl ammoniumhydroxide or chloride, triethylphenyl ammonium hydroxide, triethylphenylammonium hydroxide, triethylphenyl ammonium chloride,diethylaminoethylmethacrylate, water-soluble complex resinous aminecondensation products, such as the water soluble condensation product ofdiethanol amine and adipic acid, such as poly(adipicacid-co-methylaminoethanol), water soluble condensation products ofethylene oxide, urea, and formaldehyde and polyethyleneimine; gelatin,glue, casein, albumin, gluten, and methoxycellulose. The preferredpromoter is triethylphenyl ammonium chloride.

In order to prevent the hydrophilic colloid from removing the suspensionstabilizing agent from the surface of the droplets, suitable anionicsurfactants may be included in the mixing step to prepare the coatingcomposition such as polyisopropyl naphthalene-sodium sulfonate, sodiumdodecyl sulfate, sodium dodecyl benzene sulfonate, as well as thoseanionic surfactants set forth in U.S. Pat. No. 5,326,687 and in SectionXI of Research Disclosure 308119, December 1989, entitled “PhotographicSilver Halide Emulsions, Preparations, Addenda, Processing, andSystems”, both of which are incorporated herein by reference. Aromaticsulfonates are more preferred and polyisopropyl naphthalene sulfonate ismost preferred.

Suitable hydrophilic binders for use in the liquid crystal layer mayinclude both naturally occurring substances such as proteins, proteinderivatives, cellulose derivatives (e.g. cellulose esters), gelatins andgelatin derivatives, polysaccaharides, casein, and the like, andsynthetic water permeable colloids such as poly(vinyl lactams),acrylamide polymers, poly(vinyl alcohol) and its derivatives, hydrolyzedpolyvinyl acetates, polymers of alkyl and sulfoalkyl acrylates andmethacrylates, polyamides, polyvinyl pyridine, acrylic acid polymers,maleic anhydride copolymers, polyalkylene oxide, methacrylamidecopolymers, polyvinyl oxazolidinones, maleic acid copolymers, vinylamine copolymers, methacrylic acid copolymers, acryloyloxyalkyl acrylateand methacrylates, vinyl imidazole copolymers, vinyl sulfide copolymers,and homopolymer or copolymers containing styrene sulfonic acid. Gelatinis preferred.

Modern chiral nematic liquid crystal materials usually include at leastone nematic host combined with a chiral dopant. In general, the nematicliquid crystal phase is composed of one or more mesogenic componentscombined to provide useful composite properties. Many such materials areavailable commercially. The nematic component of the chiral nematicliquid crystal mixture may be comprised of any suitable nematic liquidcrystal mixture or composition having appropriate liquid crystalcharacteristics. Nematic liquid crystals suitable for use in the presentinvention are preferably composed of compounds of low molecular weightselected from nematic or nematogenic substances, for example from theknown classes of the azoxybenzenes, benzylideneanilines, biphenyls,terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl estersof cyclohexanecarboxylic acid; phenyl or cyclohexyl esters ofcyclohexylbenzoic acid; phenyl or cyclohexyl esters ofcyclohexylcyclohexanecarboxylic acid; cyclohexylphenyl esters of benzoicacid, of cyclohexanecarboxyiic acid and ofcyclohexylcyclohexanecarboxylic acid; phenyl cyclohexanes;cyclohexylbiphenyls; phenyl cyclohexylcyclohexanes;cyclohexylcyclohexanes; cyclohexylcyclohexenes;cyclohexylcyclohexylcyclohexenes; 1,4-bis-cyclohexylbenzenes;4,4-bis-cyclohexylbiphenyls; phenyl- or cyclohexylpyrimidines; phenyl-or cyclohexylpyridines; phenyl- or cyclohexylpyridazines; phenyl- orcyclohexyldioxanes; phenyl- or cyclohexyl-1,3-dithianes;1,2-diphenylethanes; 1,2-dicyclohexylethanes;1-phenyl-2-cyclohexylethanes;1-cyclohexyl-2-(4-phenylcyclohexyl)ethanes;1-cyclohexyl-2′,2-biphenylethanes; 1-phenyl-2-cyclohexylphenylethanes;optionally halogenated stilbenes; benzyl phenyl ethers; tolanes;substituted cinnamic acids and esters; and further classes of nematic ornematogenic substances. The 1,4-phenylene groups in these compounds mayalso be laterally mono- or difluorinated. The liquid crystallinematerial of this preferred embodiment is based on the achiral compoundsof this type. The most important compounds, that are possible ascomponents of these liquid crystalline materials, can be characterizedby the following formula R′—X—Y-Z-R″ wherein X and Z, which may beidentical or different, are in each case, independently from oneanother, a bivalent radical from the group formed by -Phe-, -Cyc-,-Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, —B-Phe- and —B-Cyc-;wherein Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cycis trans-1,4-cyclohexylene or 1,4-cyclohexenylene, Pyr ispyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5-diyl,and B is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5-diyl,pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl. Y in these compounds isselected from the following bivalent groups —CH═CH—, —C≡C—, —N═N(O)—,—CH═CY′—, —CH═N(O)—, —CH2-CH2-, —CO—O—, —CH2-O—, —CO—S—, —CH2—S—,—COO-Phe-COO— or a single bond, with Y′ being halogen, preferablychlorine, or —CN; R′ and R″ are, in each case, independently of oneanother, alkyl, alkenyl, alkoxy, alkenyloxy, alkanoyloxy, alkoxycarbonylor alkoxycarbonyloxy with 1 to 18, preferably 1 to 12 C atoms, oralternatively one of R′ and R″ is —F, —CF3, —OCF3, —Cl, —NCS or —CN. Inmost of these compounds R′ and R′ are, in each case, independently ofeach another, alkyl, alkenyl or alkoxy with different chain length,wherein the sum of C atoms in nematic media generally is between 2 and9, preferably between 2 and 7. The nematic liquid crystal phasestypically consist of 2 to 20, preferably 2 to 15 components. The abovelist of materials is not intended to be exhaustive or limiting. Thelists disclose a variety of representative materials suitable for use ormixtures, which comprise the active element in electro-optic liquidcrystal compositions.

Suitable chiral nematic liquid crystal compositions preferably have apositive dielectric anisotropy and include chiral material in an amounteffective to form focal conic and twisted planar textures. Chiralnematic liquid crystal materials are preferred because of theirexcellent reflective characteristics, bi-stability and gray scalememory. The chiral nematic liquid crystal is typically a mixture ofnematic liquid crystal and chiral material in an amount sufficient toproduce the desired pitch length. Suitable commercial nematic liquidcrystals include, for example, E44, E48, E31, E80, BL087, BL101,ZLI-3308, ZLI-3273, ZLI-5048-000, ZLI-5049-100, ZLI-5100-100,ZLI-5800-000, MLC-6041-100.TL202, TL203, TL204 and TL205 manufactured byE. Merck (Darmstadt, Germany). Although nematic liquid crystals havingpositive dielectric anisotropy, and especially cyanobiphenyls, arepreferred, virtually any nematic liquid crystal known in the art,including those having negative dielectric anisotropy should be suitablefor use in the invention. Other nematic materials may also be suitablefor use in the present invention as would be appreciated by thoseskilled in the art.

The chiral dopant added to the nematic mixture to induce the helicaltwisting of the mesophase, thereby allowing reflection of visible light,can be of any useful structural class. The choice of dopant depends uponseveral characteristics including among others its chemicalcompatibility with the nematic host, helical twisting power, temperaturesensitivity, and light fastness. Many chiral dopant classes are known inthe art: e.g., G. Gottarelli and G. Spada, Mol. Cryst. Liq. Crys., 123,377 (1985); G. Spada and G. Proni, Enantiomer, 3, 301 (1998) andreferences therein. Typical well-known dopant classes include1,1-binaphthol derivatives; isosorbide (D-1) and similar isomannideesters as disclosed in U.S. Pat. No. 6,217,792; TADDOL derivatives (D-2)as disclosed in U.S. Pat. No. 6,099,751; and the pending spiroindanesesters (D-3) as disclosed in U.S. patent application Ser. No. 10/651,692by T. Welter et al., filed Aug. 29, 2003, titled “Chiral Compounds AndCompositions Containing The Same,” hereby incorporated by reference.

The pitch length of the liquid crystal materials may be adjusted basedupon the following equation (1):λ_(max) =n _(av) p ₀where λ_(max) is the peak reflection wavelength, that is, the wavelengthat which reflectance is a maximum, n_(av) is the average index ofrefraction of the liquid crystal material, and p₀ is the natural pitchlength of the chiral nematic helix. Definitions of chiral nematic helixand pitch length and methods of its measurement, are known to thoseskilled in the art such as can be found in the book, Blinov, L. M.,Electro-optical and Magneto-Optical Properties of Liquid Crystals, JohnWiley & Sons Ltd. 1983. The pitch length is modified by adjusting theconcentration of the chiral material in the liquid crystal material. Formost concentrations of chiral dopants, the pitch length induced by thedopant is inversely proportional to the concentration of the dopant. Theproportionality constant is given by the following equation (2):p ₀=1/(HTP.c)where c is the concentration of the chiral dopant and HTP is theproportionality constant.

For some applications, it is desired to have LC mixtures that exhibit astrong helical twist and thereby a short pitch length. For example inliquid crystalline mixtures that are used in selectively reflectingchiral nematic displays, the pitch has to be selected such that themaximum of the wavelength reflected by the chiral nematic helix is inthe range of visible light. Other possible applications are polymerfilms with a chiral liquid crystalline phase for optical elements, suchas chiral nematic broadband polarizers, filter arrays, or chiral liquidcrystalline retardation films. Among these are active and passiveoptical elements or color filters and liquid crystal displays, forexample STN, TN, AMD-TN, temperature compensation, polymer free orpolymer stabilized chiral nematic texture (PFCT, PSCT) displays.Possible display industry applications include ultralight, flexible, andinexpensive displays for notebook and desktop computers, instrumentpanels, video game machines, videophones, mobile phones, hand-held PCs,PDAs, e-books, camcorders, satellite navigation systems, store andsupermarket pricing systems, highway signs, informational displays,smart cards, toys, and other electronic devices. The fluid-coatedconductive layer may also find application in electroluminescentdisplays.

Chiral nematic liquid crystal materials and cells, as well as polymerstabilized chiral nematic liquid crystals and cells, are well known inthe art and described in, for example, co-pending application Ser. No.07/969,093 filed Oct. 30, 1992; Ser. No. 08/057,662 filed May 4, 1993;Yang et al., Appl. Phys. Lett. 60(25) pp 3102-04 (1992); Yang et al., J.Appl. Phys. 76(2) pp 1331 (1994); published International PatentApplication No. PCT/US92/09367; and published International PatentApplication No. PCT/US92/03504, all of which are incorporated herein byreference.

Liquid crystalline materials, often referred to as emulsions, may bemachine coatable using coating equipment of the type employed in themanufacture of photographic films.

The flexible plastic substrate can be any flexible self-supportingplastic film that supports the thin conductive metallic film. “Plastic”means a high polymer, usually made from polymeric synthetic resins,which may be combined with other ingredients, such as curatives,fillers, reinforcing agents, colorants, and plasticizers. Plasticincludes thermoplastic materials and thermosetting materials.

The flexible plastic film must have sufficient thickness and mechanicalintegrity so as to be self-supporting, yet should not be so thick as tobe rigid. Typically, the flexible plastic substrate is the thickestlayer of the composite film in thickness. Consequently, the substratedetermines to a large extent the mechanical and thermal stability of thefully structured composite film.

Another significant characteristic of the flexible plastic substratematerial is its glass transition temperature (Tg). Tg is defined as theglass transition temperature at which plastic material will change fromthe glassy state to the rubbery state. It may comprise a range beforethe material may actually flow. Suitable materials for the flexibleplastic substrate include thermoplastics of a relatively low glasstransition temperature, for example up to 150° C., as well as materialsof a higher glass transition temperature, for example, above 150° C. Thechoice of material for the flexible plastic substrate would depend onfactors such as manufacturing process conditions, such as depositiontemperature, and annealing temperature, as well as post-manufacturingconditions such as in a process line of a displays manufacturer. Certainof the plastic substrates discussed below can withstand higherprocessing temperatures of up to at least about 200° C., some up to3000-350° C., without damage.

Typically, the flexible plastic substrate is polyethylene terephthalate(PET), polyethylene naphthalate (PEN), polyethersulfone (PES),polycarbonate (PC), polysulfone, a phenolic resin, an epoxy resin,polyester, polyimide, polyetherester, polyetheramide, cellulose acetate,aliphatic polyurethanes, polyacrylonitrile, polytetrafluoroethylenes,polyvinylidene fluorides, poly(methyl(x-methacrylates), an aliphatic orcyclic polyolefin, polyarylate (PAR), polyetherimide (PEI),polyethersulphone (PES), polyimide (PI), Teflonpoly(perfluoro-alboxy)fluoropolymer (PFA), poly(ether ether ketone)(PEEK), poly(ether ketone) (PEK), poly(ethylenetetrafluoroethylene)fluoropolymer (PETFE), and poly(methyl methacrylate)and various acrylate/methacrylate copolymers (PMMA). Aliphaticpolyolefins may include high density polyethylene (HDPE), low densitypolyethylene (LDPE), and polypropylene, including oriented polypropylene(OPP). Cyclic polyolefins may include poly(bis(cyclopentadiene)). Apreferred flexible plastic substrate is a cyclic polyolefin or apolyester. Various cyclic polyolefins are suitable for the flexibleplastic substrate. Examples include Arton® made by Japan SyntheticRubber Co., Tokyo, Japan; Zeanor T made by Zeon Chemicals L.P., TokyoJapan; and Topas® made by Celanese A. G., Kronberg Germany. Arton is apoly(bis(cyclopentadiene)) condensate that is a film of a polymer.Alternatively, the flexible plastic substrate can be a polyester. Apreferred polyester is an aromatic polyester such as Arylite. Althoughvarious examples of plastic substrates are set forth above, it should beappreciated that the substrate can also be formed from other materialssuch as glass and quartz.

The flexible plastic substrate can be reinforced with a hard coating.Typically, the hard coating is an acrylic coating. Such a hard coatingtypically has a thickness of from 1 to 15 microns, preferably from 2 to4 microns and can be provided by free radical polymerization, initiatedeither thermally or by ultraviolet radiation, of an appropriatepolymerizable material. Depending on the substrate, different hardcoatings can be used. When the substrate is polyester or Arton, aparticularly preferred hard coating is the coating known as “Lintec”.Lintec contains UV-cured polyester acrylate and colloidal silica. Whendeposited on Arton, it has a surface composition of 35 atom % C, 45 atom% 0, and 20 atom % Si, excluding hydrogen. Another particularlypreferred hard coating is the acrylic coating sold under the trademark“Terrapin” by Tekra Corporation, New Berlin, Wis.

The coated conductive layer of the present invention may be anyconductive layer. In one embodiment, the coated conductive layer may bethe layer referred to as the first conductive layer. For example, in oneembodiment, the light-modulating layer may be coated over a transparentfirst conductive layer on a polyester display substrate and dried toprovide an approximately 9-micrometer thick polymer dispersedcholesteric coating. In other embodiments, the coated conductive layermay be referred to the second (or greater) conductive layer. Both thefirst and second (or greater) conductive layers may comprisefluid-coated conductive material compositions. For example, a displaymay comprise a flexible support having a thin first transparentconductors applied thereto, after which a machine-coated cholestericliquid-crystal layer is applied to the first transparent conductor. Amachine coated second conductor, applied to the cholesteric liquidcrystal layer permits the fabrication of a low-cost flexible display.Small displays according to the present invention can be used aselectronically rewritable tags or labels for inexpensive, rewriteapplications.

Other types of conductive layers may be incorporated into displaysaccording to the present invention. These other types of conductivelayers may comprise a primary metal oxide, such as indium oxide,titanium dioxide, cadmium oxide, gallium indium oxide, niobium pentoxideand tin dioxide. See, Int. Publ. No. WO 99/36261 by PolaroidCorporation. In addition to the primary oxide, the other conductivelayer may also comprise a secondary metal oxide such as an oxide ofcerium, titanium, zirconium, hafnium and/or tantalum. See, U.S. Pat. No.5,667,853 to Fukuyoshi et al. (Toppan Printing Co.) Other transparentconductive oxides include, but are not limited to ZnO₂, Zn₂SnO₄,Cd₂SnO₄, Zn₂In₂O₅, MgIn₂O₄, Ga₂O₃—In₂O₃, or TaO₃. The other conductivelayer or layers may be formed, for example, by a low temperaturesputtering technique or by a direct current sputtering technique, suchas DC-sputtering or RF-DC sputtering, depending upon the material ormaterials of the underlying layer. The conductive layer may be atransparent, electrically conductive layer of tin-oxide orindium-tin-oxide (ITO), or a coated organic conductor such aspolythiophene. Typically, the other conductive layer (or layers) issputtered onto the substrate to a resistance of less than 250 ohms persquare. Alternatively, the other conductive layer may be an opaqueelectrical conductor formed of metal such as copper, aluminum or nickel.If the other conductive layer is an opaque metal, the metal can be ametal oxide to create a light absorbing conductive layer.

Indium tin oxide (ITO) is a preferred other conductive material, as itis a cost effective conductor with good environmental stability, up to90% transmission, and down to 20 ohms per square resistivity. Anexemplary preferred ITO layer has a % transmittance (% T) greater thanor equal to 80% in the visible region of light, that is, from greaterthan 400 nm to 700 nm, so that the film will be useful for displayapplications. In a preferred embodiment, the other conductive layercomprises a layer of low temperature ITO that is polycrystalline. TheITO layer is preferably 10-120 nm in thickness, or more preferably50-100 nm thick to achieve a resistivity of 20-60 ohms/square onplastic. An exemplary preferred ITO layer is 60-80 nm thick.

The conductive layer or layers are preferably patterned. The conductivelayer or layers may preferably be patterned into a plurality ofelectrodes. The patterned electrodes may be used to form a LCD device.In another embodiment, two conductive substrates are positioned facingeach other and cholesteric liquid crystals are positioned therebetweento form a device. The patterned conductive layer or layers may have avariety of dimensions. Exemplary dimensions may include line widths of10 microns, distances between lines, that is, electrode widths, of 200microns, depth of cut, that is, thickness of the conductor, of 100nanometers. Thicknesses on the order of 60, 70, and greater than 100nanometers are also possible.

The conductive layers may also be patterned by irradiating themultilayered conductor/substrate structure with ultraviolet radiation sothat portions of the conductive layer are ablated therefrom. It is alsoknown to employ an infra-red (IR) fiber laser for patterning a metallicconductive layer overlying a plastic film, directly ablating theconductive layer by scanning a pattern over the conductor/filmstructure. See: Int. Publ. No. WO 99/36261 and “42.2: A New ConductorStructure for Plastic LCD Applications Utilizing ‘All Dry’ Digital LaserPatterning,” 1998 SID International Symposium Digest of TechnicalPapers, Anaheim, Calif., May 17-22, 1998, no. VOL. 29, May 17, 1998,pages 1099-1101, both incorporated herein by reference.

The display may also contain still other types of conductive layers,such as printed conductive ink. For higher conductivities, these stillother conductive layers may comprise a silver-based layer which containssilver only or silver containing a different element such as aluminum(Al), copper (Cu), nickel (Ni), cadmium (Cd), gold (Au), zinc (Zn),magnesium (Mg), tin (Sn), indium (In), tantalum (Ta), titanium (Ti),zirconium (Zr), cerium (Ce), silicon (Si), lead (Pb) or palladium (Pd).In a preferred embodiment, the still other conductive layer may compriseat least one of gold, silver and a gold/silver alloy, for example, alayer of silver coated on one or both sides with a thinner layer ofgold. See, Int. Publ. No. WO 99/36261 by Polaroid Corporation. Inanother embodiment, the still other conductive layer may comprise alayer of silver alloy, for example, a layer of silver coated on one orboth sides with a layer of indium cerium oxide (InCeO). See U.S. Pat.No. 5,667,853, incorporated herein in by reference.

A gel sub-layer may be applied over the transparent conductors prior toapplying light modulating layers as disclosed in U.S. Pat. No.6,423,368, hereby incorporated by reference in its entirety.

The LCD may also comprise at least one “functional layer” betweenlayers. The functional layer may comprise a protective layer or abarrier layer. A preferred barrier layer may acts as a gas barrier or amoisture barrier and may comprise SiOx, AlOx or ITO. The protectivelayer, for example, an acrylic hard coat, functions to prevent laserlight from penetrating to functional layers between the protective layerand the substrate, thereby protecting both the barrier layer and thesubstrate. The functional layer may also serve as an adhesion promoterof the conductive layer to the substrate.

In another embodiment, the polymeric support may further comprise anantistatic layer to manage unwanted charge build up on the sheet or webduring roll conveyance or sheet finishing. Since the liquid crystal areswitched between states by voltage, charge accumulation of sufficientvoltage on the web surface may create an electrical field that whendischarged may switch a portion of the liquid crystal. It is well knownin the art of photographic web based materials that winding, conveying,slitting, chopping and finishing can cause charge build on many webbased substrates. High charge buildup is a particular problem withplastic webs that are conductive on one side but not on the other side.Charges accumulates on one side on the web to the point of discharge andin photographic light sensitive materials that discharge can result infog which is uncontrolled light exposure as a result of the spark causedfrom the discharge. Similar precaution and static management isnecessary during manufacturing or in end use applications for liquidcrystal displays. In another embodiment of this invention, theantistatic layer has a surface resistivity of between 10⁵ to 10¹². Above10¹², the antistatic layer typically does not provide sufficientconduction of charge to prevent charge accumulation to the point ofpreventing fog in photographic systems or from unwanted point switchingin liquid crystal displays. While layers greater than 10⁵ will preventcharge buildup, most antistatic materials are inherently not thatconductive and in those materials that are more conductive than 10⁵,there is usually some color associated with them that will reduce theoverall transmission properties of the display. The antistatic layer isseparate from the highly conductive layer of ITO and provides the beststatic control when it is on the opposite side of the web substrate fromthat of the ITO layer. This may include the web substrate itself.

In a preferred embodiment, a dispersion of fine, inexpensive andenvironmentally inert conductive particles, such as silver, nickel orchrome, may be coated over a layer of polymer dispersed cholestericliquid crystal and a nanoparticle non-conductive dark layer. One type ofdark layer may be a color contrast layer. Color contrast layers may beradiation reflective layers or radiation absorbing layers. In somecases, the rearmost substrate of each display may preferably be paintedblack. In the case of the stacked cell display, the contrast may beimproved by painting the back substrate of the last visible cell black.In one embodiment, a light absorber may be positioned on the sideopposing the incident light. In the fully evolved focal-conic state, thechiral nematic liquid crystal is transparent, passing incident light,which is absorbed by the light absorber to create a black image.Progressive evolution of the focal-conic state causes a viewer toperceive a reflected light that transitions to black as the chiralnematic material changes from planar state to a focal conic state. Thetransition to the light transmitting state is progressive, and varyingthe low voltage time permits variable levels of reflection. Thesevariable levels may be mapped out to corresponding gray levels, and whenthe field is removed, the light-modulating layer maintains a givenoptical state indefinitely. This process is more fully discussed in U.S.Pat. No. 5,437,811, incorporated herein by reference.

The color contrast layer may also be other colors. In anotherembodiment, the dark layer comprises milled non-conductive pigments. Thematerials are milled below 1 micron to form “nano-pigments”. Suchpigments are effective in absorbing wavelengths of light in very thin or“sub micron” layers. In a preferred embodiment, the dark layer absorbsall wavelengths of light across the visible light spectrum, that is from400 nanometers to 700 nanometers wavelength. The dark layer may alsocontain a set or multiple pigment dispersions. For example, threedifferent pigments, such as a Yellow pigment milled to median diameterof 120 nanometers, a magenta pigment milled to a median diameter of 210nanometers, and a cyan pigment, such as Sunfast® Blue Pigment 15:4pigment, milled to a median diameter of 110 nanometers are combined. Amixture of these three pigments produces a uniform light absorptionacross the visible spectrum. Suitable pigments are readily available andare designed to be light absorbing across the visible spectrum. Inaddition, suitable pigments are inert and do not carry electricalfields.

Suitable pigments used in the color contrast layer may be any coloredmaterials, which are practically insoluble in the medium in which theyare incorporated. The preferred pigments are organic in which carbon isbonded to hydrogen atoms and at least one other element such asnitrogen, oxygen and/or transition metals. The hue of the organicpigment is primarily defined by the presence of one or morechromophores, a system of conjugated double bonds in the molecule, whichis responsible for the absorption of visible light. Suitable pigmentsinclude those described in Industrial Organic Pigments: Production,Properties, Applications by W. Herbst and K. Hunger, 1993, WileyPublishers. These include, but are not limited to, Azo Pigments such asmonoazo yellow and orange, diazo, naphthol, naphthol reds, azo lakes,benzimidazolone, diazo condensation, metal complex, isoindolinone andisoindolinic, polycyclic pigments such as phthalocyanine, quinacridone,perylene, perinone, diketopyrrolo-pyrrole, and thioindigo, andanthriquinone pigments such as anthrapyrimidine, triarylcarbonium andquinophthalone.

The protective layer useful in the practice of the invention can beapplied in any of a number of well-known techniques, such as dipcoating, rod coating, blade coating, air knife coating, gravure coatingand reverse roll coating, extrusion coating, slide coating, curtaincoating, and the like. The lubricant particles and the binder arepreferably mixed together in a liquid medium to form a coatingcomposition. The liquid medium may be a medium such as water or otheraqueous solutions in which the hydrophilic colloid are dispersed with orwithout the presence of surfactants.

FIG. 1 is a perspective section view of a display in accordance with theinvention. A sheet designated as display 10 is made in accordance withthe present invention. Display 10 includes a display substrate 15, whichcan be a thin transparent polymeric material, such as Kodak Estar® filmbase formed of polyester plastic that has a thickness of between 20 and200 micrometers. In an exemplary embodiment, display substrate 15 is a125-micrometer thick sheet of polyester film base. Other polymers, suchas transparent polycarbonate, can also be used.

One or more first transparent conductors 20 are formed on displaysubstrate 15. First transparent conductors 20 can be tin-oxide,indium-tin-oxide (ITO) with ITO being the preferred material. Typicallythe material of first transparent conductors 20 is sputtered or coatedas a layer over display substrate 15 having a resistance of less than1000 ohms per square. First transparent conductors 20 can be formed inthe conductive layer by conventional lithographic or laser etchingmeans. Transparent first transparent conductors 20 can also be formed byprinting a transparent organic conductor such as PEDT/PSS, PEDOT/PSSpolymer, which materials are sold as Baytron® P by Bayer AG ElectronicChemicals. Portions of light modulating layer 30, dark layer 35 and theconductive continuum are removed, for example, using a solvent to formexposed first conductors 22.

FIG. 2 is a schematic section view of optical states of one embodimentof a display in accordance with the present invention. The left diagramdemonstrates the optical path when the cholesteric material is in aplanar state. Incident light 60 strikes planar liquid crystal 72 whichreflects a portion of incident light 60 as reflected light 62. Theremaining light passes through dark layer 35.

Dark layer 35 can be a complementary light-absorbing dye layer thatoperates on a portion of the light passing though dark layer 35.Particular wavelengths of light are absorbed, and the remaining lightstrikes reflective second conductor 40. Light is reflected from secondconductor 40 and passes through dark layer 35 a second time, then passesthrough planar material 72 to become complementary light 64.Complementary light 64, operating in conjunction with cholesteric liquidcrystal having peak reflectance near 575 nanometers, forms asubstantially color-neutral reflective surface.

On the right side of FIG. 2, the liquid-crystal material is in afocal-conic state 74 and transmits incident light 60. Dark layer 35provides complementary light 64 when the liquid crystal material is in afocal conic state. As one alternative, dark layer 35 can be a thin,black layer that absorbs across all wavelengths of light. With apanchromatic, black dark layer 35, when the cholesteric material is inthe focal-conic state, complementary light does not exist and the imagewill be essentially black.

In the present embodiment, in FIG. 1, dark layer 35 is coated over lightmodulating layer 30 to provide a light-absorbing layer that provides aspecific contrast state to reflected light. As mentioned above, darklayer 35 can be designed to provide a specific amount of light atwavelengths not operated on by the cholesteric liquid crystal to createa more color-neutral image. The coating can be simultaneous with thedeposition of light modulating layer 30 or as a separate step. In thedescribed embodiment, multi-layer coating equipment of the kind used inthe photographic industry provides light modulating layer 30 and darklayer 35 as two co-deposited layers. Dark layer 35 is significantlythinner than light modulating layer 30 and has minimal effect on theelectrical field strength required to change the state of thecholesteric liquid-crystal material.

Second conductors 40 overlay dark layer 35. Second conductors 40 havesufficient conductivity to induce an electric field across lightmodulating layer 30 strong enough to change the optical state of thepolymeric material. In the general case, the conductivity should atleast match the conductivity of the first transparent conductor, whichhas a sheet resistance of less than 100 ohms per square. In thisinvention, second conductors 40 are conductive materials that have beencoated as a fluid, dried and laser etched. The fluid can be water or anorganic solvent. A binding agent is in solution with the solvent, andfine conductive particles are suspended in the fluid. In practice, thevolume ratio between a nonconductive binder and the conductive particlesshould be less than 2 part binder to 1 part conductor.

Aqueous coatings with a gelatin binder with low gelatin concentrationshave viscosities below 100 centipoise. It has been found experimentallythat particles should have sizes of less than 5 microns and preferablyless than 2 microns to stay in suspension for a time period long enoughto permit pumping to a coater. Commercial dried silver materials withinthat size range are available from Ferro Corporation of Cleveland Ohioas SFK-ED, S7000-14, S7000-35 or S11000-25. Metalor Corporation ofNeuchatel, Switzerland including C-0083P fine silver, also suppliessilver powders useful in the application. Such materials have 90% of theparticles less than 2 microns in diameter. It is useful to filtersuspensions of such materials through filters passing only particlesbelow microns to ensure the material is delivered to the coatingequipment without precipitation.

Conductive layers useful in providing electrical fields which canoperate on electrical fields require sheet conductivities less than 500ohms per square. Conductor layers in this application refer to layershave electrical conductivity sufficient to operate on electro-opticalmaterials and have significantly lower sheet resistance than coatedanti-static layers. Second conductors 40 must be thin enough to permitlaser etching of the coated conductor without damage to first conductors20. It is preferable that second conductors be thin enough that there isno visually apparent damage to dark layer 35. The layer thickness forsecond conductor 4 is preferably below 1 micron in application. Thethinness and conductivity requirements practically limit the conductivematerial to fine silver below 2 microns in diameter.

The use of a flexible support for display substrate 15, thin firsttransparent conductors 20, machine-coated cholesteric liquid-crystallayer 30, and machine coated second conductors 40 permits thefabrication of a low-cost flexible display. Small displays according tothe present invention can be used as electronically rewritable tags orlabels for inexpensive, rewrite applications.

The fluid-coated electrically conductive layer and a display containingthe same may be formed by providing a substrate and coating thereon anelectrically conductive layer comprising a fluid and an electricallyconductive material, wherein the electrically conductive material hassufficient conductivity to induce an electric field strong enough tochange the optical state of a light modulating material. A lightmodulating material may be coated between the substrate and thefluid-coated electrically conductive layer. This may be accomplishedprior to or simultaneously with the coating of the fluid-coatedelectrically conductive layer. A patterned conductive layer may also beapplied between the substrate and the light modulating material prior tothe coating of the liquid crystal layer. A light modulating material mayalso be coated on top of the fluid-coated electrically conductive layer.In a preferred embodiment, the light modulating material comprises aliquid crystal material.

FIG. 3 is a first sectional view of fluidic deposition of a conductivelayer. Substrate 15 supports transparent first conductors 20, lightmodulating material 30 and dark layer 35. Coating block 90 is a slidecoater of conventional design. Fluid second conductor 40′ is pumpedthrough a slot 92 and flows in a laminar manner down slope 94 to fallonto dark layer 35 on the web being transported relative to coatingblock 90. Fluid second conductor 40′ can contain gelatin in solution asa binding agent. The gelatin can be chill set, and dried to form aconductive continuum that can be patterned to form second conductors 40.

FIG. 4 is a second sectional view of fluidic deposition of a conductivelayer. Substrate 15, having transparent first conductors 20, movesrelative to coating block 90. Coating block 90 is a slide coater ofconventional design. Fluid second conductor 40′ is pumped through a slot92 and flows in a laminar manner down slope 94. Fluid dark layer 35′ ispumped through a second slot 96 downstream of fluid second conductor40′. Because the two streams are laminar flow, fluid second conductor40′ lays over fluid dark layer 35′ without mixing. Fluid lightmodulating layer 30′ is pumped through a third slot 100 further downstream, and the first two layers lay over fluid light modulating layer30′ without mixing. The fluidized layers fall onto transparent secondconductors 20 on substrate 15 being transported relative to coatingblock 90. The three fluid layers can contain gelatin in solution as abinding agent. The gelatin in all three layers can be chill set anddried simultaneously, creating a conductive top layer that can bepatterned to form second conductor 40. Multiple layers, greater thanthree in number, may be deposited simultaneously or sequentially.

FIG. 5 is a first side sectional view of a display having a fluiddeposited conductive layer in the wet and dried state. On the left sideof the drawing, a fluid second conductor 40′ has been applied over otherdried layers. In the disclosed preferred embodiment, fluid secondconductor 40′ is 25 microns thick and the remaining layers areapproximately. 10 microns thick. It is useful, but not necessary, forprior layers to use gelatin as a binder and further incorporate across-linking agent to prevent disruption by the application of aaqueous based fluid second conductor 40′. Fluid second conductor 40′ isdried on the right side of FIG. 5 to create a conductive layer that canbe patterned to form second conductors 40 that are less than 1 micronthick. It is desirable that the amount of material that creates secondconductors 40 be significantly thinner and have less mass than darklayer 35 and light modulating layer 30 for laser ablation. Such layersare significantly thinner than the screen-printed materials disclosed inprior art, which are typically 15 to 25 microns thick. The thinnerconductive layer reduces the cost of material for second conductor 40.

FIG. 6 is a second side sectional view of a display having a fluiddeposited conductive layer in the wet and dried state. In thisembodiment, fluid light modulating layer 30′, fluid dark layer 35′ andfluid second conductor 40′ are deposited simultaneously in accordancewith the process shown in FIG. 4. The set of three fluid layers is onthe order of 75 to 100 microns thick. Because fluid second conductor 40′lays on top of other fluidized layers, it is desirable that the denseconductive particles within fluid second conductor 40′ not precipitateinto the other fluidized layers. Silver particles that do notprecipitate into an adjacent, fluidized layer should be under 5 micronmean diameter, and preferably under 2 micron in diameter. On the rightside of FIG. 6, all three layers have been dried simultaneously tocreate a display structure ready for final laser patterning of the topconductive layer to form second conductors 40.

FIG. 7 is a side sectional view of a dried, fluid deposited conductorbeing etched by a laser beam 104. In an experiment, fluid depositedcoatings in accordance with the present invention were etched using a 4watt Yttrium-Aluminum-Garnet (YAG) pulsed laser beam 104. Patterns werecut in the fluid deposited layer deposited for use as second conductor40 in accordance according to the preferred embodiment without cuttingthrough patterned first transparent conductor 20. In particular, thefluid deposited conductor was thin enough that dark layer 35 was notdamaged. Dark layer 35 is useful in the invention for stopping excesslevels of radiation from laser beam 104 above the energy level requiredto cut through the coated conductor layer.

The dark layer in this application can be formed of particulate materialcapable of absorbing high levels of laser energy before destruction.Dark layer 35 can incorporate, for instance, carbon or heavy metals thatrequire high levels of energy for ablation. Dark layer 35 can furtherhave reflective components that reflect rather than absorb excess laserenergy.

A process for fabricating display 10 will now be described using FIG.8-FIG. 11. FIG. 8 is a rear view of a sheet in accordance with the oneembodiment of the present invention; which sheet has a patterned firstconductor 20. A substrate 15 is provided with a plurality of patternedfirst transparent conductors 20. FIG. 9 is a rear view of a sheet inaccordance with the present invention having a polymer-dispersedcholesteric liquid-crystal light modulating layer 30, a dark layer 35and a top coated continuum of conductive material. In the describedembodiment, light modulating layer 30, dark layer 35 and the conductivecontinuum are co-deposited using a common binder, in one case gelatin,and dried. FIG. 10 is a rear view of a sheet in accordance with thepresent invention having exposed first conductors 22. Portions of lightmodulating layer 30, dark layer 35 and the conductive continuum areremoved, for example, using a solvent, such as water, to form exposedfirst conductors 22.

FIG. 11 is a rear view of a display in accordance with the presentinvention having second conductors 40 etched into fluid depositedconductive layer. A YAG laser has been used to etch second conductors40. Patterns can be of indicia, segments of a seven segment display or aset of adjacent traces that can be organized into a matrix displays, orcombinations thereof. FIG. 12 is a section view of a display inaccordance with the present invention attached to a circuit board.Contacts 80 on circuit board 82 provide electrical connection to eachsecond conductor 40 and an associated transparent first conductor 20.

FIG. 13 is a front view of a display in accordance with the presentinvention connected to electric drive means. Row driver 84 is connectedby contacts 80 to second conductors 40. Column driver 86 is connected bycontacts 80 to first transparent conductors 20. Display 10 can beorganized with a contact 80 for each second conductor 40, or in the caseof seven-segment displays, circuit board 82 can connect common segmentsof each character to a single contact 80. Display 10 can for instance bea matrix display, having a contact 80 each row of the matrix and eachtransparent first conductor 20 being connected to an output on columndriver 86 through a contact 80. Electrical signals can be applied to rowdriver 84 and column driver 86 to write images onto display 10 inaccordance with prior art.

The following examples are provided to illustrate the invention.

EXAMPLE 1

An experiment mixture was formed to demonstrate the possibility offorming the second conductor from a solution coating that has beenlaser-etched. A solution was formed consisting of:  2.86 gms. Kodakgelatin, 40% concentration  7.88 gms. Metalor C-0083P fine silver, 38.37gms. Deionized water  0.25 gms. Zonyl FSO surfactant  0.65 gms.thickening agent.

The fine silver was Metalor Corporation material C-0083P, consisting ofde-agglomerated silver particles having a size distribution of D10=0.5um, D50=1.1 um and D90=2.0 um. The fluid contained approximately 2%volume dried gelatin and 1.50% volume of silver. The solution was coatedat 24.9 cc/m² over the previously described layers and dried to form aconductive surface. An Yttrium-Aluminum-Garnet (YAG) laser emitting 4watt output power at 1064 nanometers wavelength and focused to a 70 umdiameter beam etched the coating at 1.78 meters per second withoutcutting through the transparent second conductors. The resulting secondconductors had a sheet resistance of 80 ohms per square, which supportedan electric field that switched cholesteric liquid crystal materialbetween the focal-conic and planar states. Displays using the disclosedprocesses and materials were cycled every 10 seconds between the planarand focal-conic for one week. The 30,000 cycles were judged to besufficient for certain applications. The fluid coated, dried and etchedsilver-particle second conductors had high reflectance throughout thevisible spectrum, making the layer useful as a reflective secondconductor for a dark layer acting as a color filter.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. A patternable coatable electrically conductive layer comprising a fluid-coated electrically conductive material, wherein said electrically conductive material has sufficient conductivity to induce an electric field strong enough to change the optical state of a light modulating material.
 2. The coatable conductive layer of claim 1 wherein said fluid comprises water.
 3. The coatable conductive layer of claim 1 wherein said fluid comprises organic solvent.
 4. The coatable conductive layer of claim 1 wherein said conductive material comprises particles.
 5. The coatable conductive layer of claim 1 wherein said particles comprise precious metal.
 6. The coatable conductive layer of claim 5 wherein said precious metal particles comprise silver particles.
 7. The coatable conductive layer of claim 6 wherein said silver particles have a diameter of less than 1 micron.
 8. The coatable conductive layer of claim 1 wherein said particles comprise carbon.
 9. The coatable conductive layer of claim 1 wherein said particles comprise metal flakes.
 10. The coatable conductive layer of claim 1 wherein said particles have a diameter of less than 1 micron.
 11. The coatable conductive layer of claim 1 wherein said particles have a diameter of less than 50 nm.
 12. The coatable conductive layer of claim 1 wherein said silver particles are less than 10 cubic microns across the major length.
 13. The coatable conductive layer of claim 1 wherein said particles have a size distribution having 90 percent of the particles less than 2 microns in diameter.
 14. The coatable conductive layer of claim 1 wherein said conductive material comprises organic conductor.
 15. The coatable conductive layer of claim 14 wherein said polypyrrole.
 16. The coatable conductive layer of claim 1 wherein said conductive layer comprises a conductive polymer.
 17. The coatable conductive layer of claim 16 wherein said polythiophene.
 18. The coatable conductive layer of claim 1 wherein said conductive material has a conductivity of less than 10⁴ ohms/sq.
 19. The coatable conductive layer of claim 1 further comprising a binder.
 20. The coatable conductive layer of claim 19 wherein said binder comprises gelatin.
 21. The coatable conductive layer of claim 19 wherein said binder is water soluble.
 22. A display comprising a substrate; at least one patternable coatable electrically conductive layer comprising a fluid-coated electrically conductive material, wherein said fluid-coated electrically conductive material has sufficient conductivity to induce an electric field strong enough to change the optical state of a light modulating material, and an imaging layer comprising said light modulating material disposed over said at least one patternable fluid-coated electrically conductive layer, wherein said light modulating material has a first and a second field-switched stable optical state.
 23. The display of claim 22 wherein the imaging layer contains a polymer dispersed cholesteric liquid crystal layer.
 24. The display of claim 23 wherein said cholesteric liquid crystal layer comprises fluid dispersed domains which have been dried to form said polymer dispersed cholesteric liquid crystal layer.
 25. The display of claim 22 wherein said at least one fluid-coated conductive layer is applied from an aqueous dispersion dried to form said conductive layer.
 26. The display of claim 22 wherein said fluid comprises water.
 27. The display of claim 22 wherein said fluid comprises organic solvent.
 28. The display of claim 22 wherein said conductive material comprises particles.
 29. The display of claim 22 wherein said particles comprise precious metal.
 30. The display of claim 29 wherein said precious metal particles comprise silver particles.
 31. The display of claim 30 wherein said silver particles have a diameter of less than 1 micron.
 32. The display of claim 28 wherein said particles comprise carbon.
 33. The display of claim 28 wherein said particles comprise metal flakes.
 34. The display of claim 28 wherein said particles have a diameter of less than 1 micron.
 35. The display of claim 28 wherein said particles have a diameter of less than 50 nm.
 36. The display of claim 30 wherein said silver particles are less than 10 cubic micron across the major length.
 37. The display of claim 28 wherein said particles have a size distribution having 90 percent of the particles less than 5 microns in diameter.
 38. The display of claim 22 wherein said conductive material comprises organic conductor.
 39. The display of claim 38 wherein said polypyrrole.
 40. The display of claim 22 wherein said conductive layer comprises a conductive polymer.
 41. The display of claim 40 wherein said polythiophene.
 42. The display of claim 22 further comprising a binder.
 43. The display of claim 42 wherein said binder comprises gelatin.
 44. The display of claim 42 wherein said binder is water soluble.
 45. The display of claim 22 wherein said at least one conductive layer is patterned with actinic radiation.
 46. The display of claim 22 wherein said display further comprises at least a second patternable coatable electrically conductive layer comprising a fluid-coated electrically conductive material, wherein said fluid-coated electrically conductive material has sufficient conductivity to induce an electric field strong enough to change the optical state of a light modulating material.
 47. The display of claim 22 wherein the imaging layer further comprises a radiation absorbing layer.
 48. A method for making a coatable electrically conductive layer comprising providing a substrate and coating thereon an electrically conductive layer comprising a fluid-coated electrically conductive material, wherein said fluid-coated electrically conductive material has sufficient conductivity to induce an electric field strong enough to change the optical state of a light modulating material.
 49. The method of claim 48 further comprising coating a light modulating material between said substrate and said fluid-coated electrically conductive layer.
 50. The method of claim 49 wherein said coating said light modulating material is performed prior to said coating of said fluid-coated electrically conductive layer.
 51. The method of claim 49 wherein said coating said light modulating material is performed simultaneously to said coating of said fluid-coated electrically conductive layer.
 52. The method of claim 49 further comprising applying a patterned conductive layer between said substrate and said light modulating material and prior to said coating said liquid crystal layer.
 53. The method of claim 48 further comprising coating a light modulating material on top of said fluid-coated electrically conductive layer.
 54. The method of claim 48 wherein said coating comprises slide coating.
 55. The method of claim 48 wherein said light modulating material comprises a liquid crystal material.
 56. The method of claim 48 further comprising patterning said fluid-coated electrically conductive layer.
 57. A method for making a display comprising providing a substrate, applying a patterned conductive layer thereto, coating a light modulating layer onto said conductive layer and coating thereon a coatable electrically conductive layer comprising a fluid-coated electrically conductive material, wherein said fluid-coated electrically conductive material has sufficient conductivity to induce an electric field strong enough to change the optical state of a light modulating material.
 58. The method of claim 55 further comprising coating a light modulating material between said substrate and said fluid-coated electrically conductive layer.
 59. The method of claim 58 wherein said coating said light modulating material is performed prior to said coating of said fluid-coated electrically conductive layer.
 60. The method of claim 58 wherein said coating said light modulating material is performed simultaneously to said coating of said fluid-coated electrically conductive layer.
 61. The method of claim 58 further comprising applying a conductive layer between said substrate and said light modulating material and prior to said coating said liquid crystal layer.
 62. The method of claim 57 further comprising coating a light modulating material on top of said fluid-coated electrically conductive layer.
 63. The method of claim 57 wherein said coating comprises slide coating.
 64. The method of claim 57 wherein said light modulating material comprises a liquid crystal material.
 65. The method of claim 57 further comprising patterning said fluid-coated electrically conductive layer. 